This invention relates to fluid sampling methods and apparatus for use in a borehole in an earth formation, for obtaining samples of the formation fluids in the earth formation.
When a borehole is drilled into an earth formation in search of hydrocarbons, the borehole is typically filled with borehole fluids, primarily the re-circulating drilling fluid, or xe2x80x9cdrilling mudxe2x80x9d, used to lubricate the drill bit and carry away the cuttings. These borehole fluids penetrate into the region of the formation immediately surrounding the borehole, creating an xe2x80x9cinvaded zonexe2x80x9d that may be several tens of centimetres in radial extent.
When it is subsequently desired to obtain a sample of the formation fluids for analysis, a tool incorporating a sampling probe is lowered into the borehole (which is typically still filled with borehole fluids) to the desired depth, the sampling probe is urged against the borehole wall, and a sample of the formation fluids is drawn into the tool. However, since the sample is drawn through the invaded zone, and the tool incorporating the sampling probe is still surrounded by borehole fluids, the sample tends to become contaminated with borehole fluids from the invaded zone, and possibly even from the borehole itself, and is therefore not truly representative of the formation fluids.
One way of addressing this problem is disclosed in International Patent Application No. WO 00/43812, and involves using a sampling probe having an outer zone surrounding an inner zone, fluid being drawn into both zones. The outer zone tends to shield the inner zone from the borehole fluids surrounding the tool embodying the sample probe, and thus makes it possible to obtain a relatively uncontaminated sample of the formation fluids via the inner zone.
However, the time taken to obtain a large enough sample having a given relatively low level of contamination can vary widely in dependence on borehole conditions. It is therefore an object of the present invention in some of its aspects to alleviate this problem.
According to a first aspect of the present invention, there is provided a method of sampling the formation fluids in an earth formation surrounding a borehole, the region of the formation immediately surrounding the borehole being at least partially invaded by borehole fluids, using a borehole tool which is adapted to be lowered into the borehole and which is provided with a sampling probe device and means for urging the sampling probe device into contact with the borehole wall, the sampling probe device comprising an inner probe and an outer probe surrounding the inner probe for withdrawing respective fluid samples from the formation, wherein the ratio between the respective flow areas of the inner and outer probes is selected so as to tend to reduce the time taken to obtain via the inner probe a sample of the formation fluids having a given level of contamination by borehole fluids.
The selecting step is preferably performed in dependence upon at least one parameter selected from the radial depth of the invaded region of the formation around the borehole, the ratio between the viscosity of the borehole fluids which have invaded the formation and the viscosity of the formation fluids, and the permeability and the anisotropy of the formations.
In one implementation of the first aspect of the invention, the selecting step comprises adapting the tool to receive interchangeable sampling probe devices, and choosing the sampling probe device from among a plurality of sampling probe devices each having a different value of said ratio. In another implementation of the invention, the selecting step comprises adapting the sampling probe device to receive interchangeable inner probes, and choosing the inner probe from among a plurality of inner probes each having a different flow area.
According to a second aspect of the invention, there is provided apparatus for implementing the method of the first aspect of the invention, the apparatus comprising a borehole tool adapted to be lowered into a borehole, the tool being adapted to receive any one of a plurality of interchangeable sampling probe devices and including means for urging a received sampling probe device into contact with the borehole wall, each sampling probe device comprising an inner probe and an outer probe surrounding the inner probe for withdrawing respective fluid samples from the formation, the ratio between the respective flow areas of the inner and outer probes being different for each sampling probe device.
According to a third aspect of the invention, there is provided another apparatus for implementing the method of the first aspect of the invention, the apparatus comprising a borehole tool which is adapted to be lowered into a borehole and which is provided with a sampling probe device and means for urging the sampling probe device into contact with the borehole wall, the sampling probe device comprising an inner probe and an outer probe surrounding the inner probe for withdrawing respective fluid samples from the formation, wherein the sampling probe device is adapted to receive any one of a plurality of inner probes each having a different flow area.
In this third aspect of the invention, said inner and outer probes are advantageously substantially circular in cross-section and substantially coaxial with each other, and each said inner probe may be adapted for screw-threaded engagement with the sampling probe device.
According to a fourth aspect of the invention, there is provided a method of sampling the formation fluids in an earth formation surrounding a borehole, the region of the formation immediately surrounding the borehole being at least partially invaded by borehole fluids, using a borehole tool which is adapted to be lowered into the borehole and which is provided with a sampling probe device and means for urging the sampling probe device into contact with the borehole wall, the sampling probe device comprising an inner probe and an outer probe surrounding the inner probe for withdrawing respective fluid samples from the formation, the method comprising adjusting the ratio between the respective flow areas of the inner and outer probes so as to tend to reduce the time taken to obtain via the inner probe a sample of the formation fluids having a given level of contamination by borehole fluids.
In a preferred implementation of this fourth aspect of the invention, the adjusting step is performed in dependence upon at least one parameter selected from the radial depth of the invaded region of the formation around the borehole, the ratio between the viscosity of the borehole fluids which have invaded the formation and the viscosity of the formation fluids, and the permeability and the anisotropy of the formations, and may comprise changing the area of the end of the inner probe in contact with the wall of the borehole.
The end of the inner probe in contact with the wall of the borehole may be deformable, in which case the changing step may comprise varying the force with which said inner probe is urged into contact with the wall of the borehole. Alternatively, the inner probe may comprises a plurality of closely-fitting, coaxially-internested, relatively slideable cylinders, and the changing step may comprise varying the number of said cylinders in contact with the formation.
According to a fifth aspect of the invention, there is provide apparatus for sampling the formation fluids in an earth formation surrounding a borehole, the region of the formation immediately surrounding the borehole being at least partially invaded by borehole fluids, the apparatus comprising a borehole tool which is adapted to be lowered into the borehole and which is provided with a sampling probe device and means for urging the sampling probe device into contact with the borehole wall, the sampling probe device comprising an inner probe and an outer probe surrounding the inner probe for withdrawing respective fluid samples from the formation, and means for adjusting the ratio between the respective flow areas of the inner and outer probes so as to tend to reduce the time taken to obtain via the inner probe a sample of the formation fluids having a given level of contamination by borehole fluids.
Advantageously, the adjusting means is operated to adjust the ratio between the respective flow areas of the inner and outer probes in dependence upon at least one parameter selected from the radial depth of the invaded region of the formation around the borehole, the ratio between the viscosity of the borehole fluids which have invaded the formation and the viscosity of the formation fluids, and the permeability and the anisotropy of the formations.
Conveniently, the adjusting means comprises means for changing the area of the end of the inner probe in contact with the wall of the borehole. Thus the end of the inner probe in contact with the wall of the borehole may be deformable, and the changing means may comprise means for varying the force with which said inner probe is urged into contact with the wall of the borehole. Alternatively, the inner probe may comprise a plurality of closely-fitting, coaxially-internested, relatively slideable cylinders, and the changing means may comprise means for varying the number of said cylinders in contact with the formation.
In another implementation of the fifth aspect of the invention, the outer probe comprises an inner region, and an outer region surrounding the inner region, for withdrawing respective fluid samples from the formation, the tool further comprising valve means selectively operable to combine the fluid sample withdrawn via said inner region of the outer probe with the fluid sample withdrawn via the inner probe.
According to a sixth aspect of the invention, there is provided apparatus for sampling the formation fluids in an earth formation surrounding a borehole, the region of the formation immediately surrounding the borehole being at least partially invaded by borehole fluids, the apparatus comprising a borehole tool which is adapted to be lowered into the borehole and which is provided with a sampling probe device and means for urging the sampling probe device into contact with the borehole wall, the sampling probe device comprising an inner probe, an intermediate probe surrounding the inner probe, and an outer probe surrounding the intermediate probe, all for withdrawing respective fluid samples from the formation, the tool further comprising valve means selectively operable to combine the fluid sample withdrawn via said intermediate probe with the fluid sample withdrawn via the inner probe.